1. Field of the Invention
The present invention relates generally to an air-fuel ratio control system for an internal combustion engine, and more specifically, to the air-fuel ratio control system, wherein an air-fuel ratio feedback control is performed based on outputs from a pair of sensors which are provided on the upstream and downstream sides of a catalytic converter in an exhaust passage for monitoring the exhaust gas passing therethrough to respectively detect air-fuel ratios of an air-fuel mixture which has caused the monitored exhaust gas.
Hereinafter, for simplification of explanation, the expression "air-fuel ratio" will be used to represent not only "an air-fuel ratio of an air-fuel mixture to be fed to the engine", but also other meanings where the context allows. For example, the expression "air-fuel ratio" will also represent "an air-fuel ratio indicative or related condition of the monitored exhaust gas" or "a converted value of an air-fuel ratio", depending on the context.
2. Description of the Prior Art
Japanese First (unexamined) Patent Publication No. 2-238147 discloses an air-fuel ratio control system for an internal combustion engine of the above-noted two sensor type.
In the system of this publication, oxygen concentration sensors (hereinafter referred to as "O.sub.2 sensors") are respectively arranged on the upstream and downstream sides of a catalytic converter. FIG. 52 shows a time chart of an air-fuel ratio correction coefficient FAF and an output voltage VOX2 of the downstream O.sub.2 sensor derived in this conventional system. Specifically, when it is determined based on an output voltage of the upstream O.sub.2 sensor that an air-fuel ratio of the exhaust gas is deviated or fluctuated to a rich or lean side with respect to a stoichiometric air-fuel ratio, the air-fuel ratio correction coefficient FAF is corrected by a preset integral amount KIR or KIL in a direction opposite to that of the deviation. Further, when the monitored air-fuel ratio is inverted from rich to lean or from lean to rich across the stoichiometric air-fuel ratio, the air-fuel ratio correction coefficient FAF is corrected in a skipped manner by a skip amount RSR or RSL which is set to a value greater than the integral amount KIR or KIL, in a direction opposite to that of the deviation, so as to converge the actual air-fuel ratio to the stoichiometric air-fuel ratio. Moreover, when the output voltage VOX2 of the downstream O.sub.2 sensor largely fluctuates beyond a preset rich side limit value VRL or a preset lean side limit value VLL, the skip amount RSR or RSL is increased so as to largely correct the air-fuel ratio correction coefficient FAF for completing the correction of the air-fuel ratio as quick as possible.
Japanese First (unexamined) Patent Publication No. 3-185244 or U.S. Pat. No. 5,090,199 which is equivalent thereto, discloses another air-fuel ratio control system for an internal combustion engine of the two sensor type.
In the system of this publication, an air-fuel ratio sensor is arranged upstream of a catalytic converter, and an O.sub.2 sensor is arranged downstream of the catalytic converter. FIG. 53 shows a time chart of an output voltage VOX2 of the O.sub.2 sensor and a target air-fuel ratio .lambda.TG derived in this conventional system. Specifically, when it is determined based on the output voltage VOX2 of the O.sub.2 sensor that an air-fuel ratio of the exhaust gas is deviated to a rich or lean side with respect to the stoichiometric air-fuel ratio, the target air-fuel ratio .lambda.TG is corrected at a constant speed by a preset rich integral amount .lambda.IR or a preset lean integral amount .lambda.IL in a direction opposite to that of the deviation. Subsequently, the air-fuel ratio correction coefficient FAF is calculated at a given updating speed based on a differential or deviation between the corrected target air-fuel ratio and the actual air-fuel ratio monitored by the air-fuel ratio sensor so as to converge the actual air-fuel ratio to the stoichiometric air-fuel ratio.
However, the foregoing conventional systems have the following disadvantages, respectively:
In the system of FIG. 52, as described above, the skip amount RSR or RSL which is used at a timing determined by the output voltage of the upstream O.sub.2 sensor, is increased or decreased based on the output voltage VOX2 of the downstream O.sub.2 sensor. Accordingly, the correction of the skip amount RSR or RSL effected by the output voltage of the downstream O.sub.2 sensor is only reflected on the air-fuel ratio correction coefficient FAF when the air-fuel ratio monitored by the upstream O.sub.2 sensor crosses the stoichiometric air-fuel ratio, i.e. at a timing when the skip amount RSR or RSL is used. As a result, assuming that the downstream O.sub.2 sensor detects the air-fuel ratio as exceeding the rich side limit value VRL at a time point A in FIG. 52, the air-fuel ratio correction coefficient FAF is actually corrected by the increased lean skip amount RSL at a largely delayed time point B. This correction delay is likely to cause an excessive correction to periodically fluctuate the air-fuel ratio between the rich and lean sides so that the convergence to the stoichiometric air-fuel ratio is not effectively realized, resulting in alternate emissions of CO and HC, and NOx. Further, the correction delay is likely to cause the catalytic converter to be saturated so that the catalytic converter emits CO and HC, or NOx.
On the other hand, in the system of FIG. 53, since the air-fuel ratio correction coefficient FAF is calculated at a given updating speed based on the deviation of the actual air fuel ratio monitored by the air-fuel ratio sensor relative to the target air-fuel ratio corrected based on the output voltage VOX2 of the O.sub.2 sensor, the rich integral amount .lambda.IR or the lean integral amount .lambda.IL is immediately reflected on the air-fuel ratio correction coefficient FAF. However, since the internal combustion engine, including the three way catalytic converter, is the system which basically represents a large delay, assuming that the inversion between rich and lean is detected based on the output voltage VOX2 of the downstream O.sub.2 sensor, the air-fuel ratio upstream of the catalytic converter has already been largely deviated from the stoichiometric air-fuel ratio toward the rich or lean side. Accordingly, the delicate correction effected by the rich integral amount .lambda.IR or the lean integral amount .lambda.IL which is set very small, can not provide an effective target air-fuel ratio for quickly converging the actual air-fuel ratio to the stoichiometric air-fuel ratio. As a result, as in the conventional system of FIG. 52, the correction delay is caused so that the convergence to the stoichiometric air-fuel ratio is not realized, leading to alternate emissions of CO and HC, and NOx. Further, this correction delay is also likely to cause the catalytic converter to be saturated so that the same problem rises as in the conventional system of FIG. 52.